Aromatic carbonate is useful as a raw material for producing aromatic polycarbonate which is recently becoming more and more important as an engineering plastic, without using poisonous phosgene, or as a raw material for producing isocyanate without using poisonous phosgene. Referring to the method of producing an aromatic carbonate, a method is known, which comprises subjecting dialkyl carbonate, alkylaryl carbonate or a mixture thereof which is a starting material and an aromatic monohydroxy compound, an alkylaryl carbonate or a mixture thereof which is a reactant to an transesterification to produce a corresponding aromatic carbonate or an aromatic carbonate mixture.
However, all such transesterification are an equilibrium reaction, and due to the reactant-favored equilibrium and its low reaction rate, industrial production of aromatic carbonate by this method has been very difficult. Several approaches have been proposed in order to make this method improvements, and most of them relate to catalysts for increasing the reaction rate and a number of metal-containing catalysts are known. In a method for producing alkylaryl carbonate, diaryl carbonate or a mixture thereof by a reaction between dialkyl carbonate and an aromatic hydroxy compound, the following catalysts are proposed as such catalysts: Lewis acid such as transition metal halide or a compound which produces Lewis acid (Patent Document 1, Patent Document 2, Patent Document 3 (which is corresponding to Patent Documents 4 to 6)), tin compounds such as organic tin alkoxide and organic tin oxide (Patent Document 7 (which is corresponding to Patent Document 8)), Patent Document 9, Patent Document 10 (which is corresponding to Patent Document 11), Patent Document 12 (which is corresponding to Patent Document 13), Patent Document 14, and Patent Document 15), salts and alkoxides of alkali metal or alkaline earth metal (Patent Document 16), lead compounds (Patent Document 16), complexes of metal such as copper, iron or zirconium (Patent Document 17), titanates (Patent Document 18 which is corresponding to Patent Document 19), a mixture of Lewis acid and protonic acid (Patent Document 20 which is corresponding to Patent Document 21), a compound of Sc, Mo, Mn, Bi, Te (Patent Document 22) and ferric acetate (Patent Document 23).
In addition, in a method of producing diaryl carbonate by disproportionation of diaryl carbonate and diaryl carbonate by an transesterification between the same kind of molecules of alkylaryl carbonate, the following catalysts are proposed as such catalysts: Lewis acid and a transition metal compound which can produce Lewis acid (Patent Document 24 which is corresponding to Patent Documents 25 and 26), a polymeric tin compound (Patent Document 27 which is corresponding to Patent Document 28), a compound represented by the formula R—X(═O)OH (in which X is selected form Sn and Ti and R is selected from monovalent hydrocarbon groups) (Patent Document 29 which is corresponding to Patent Document 30), a mixture of Lewis acid and protonic acid (Patent Document 31 which is corresponding to Patent Document 32), a lead catalyst (Patent Document 33), a titanium or zirconium compound (Patent Document 34), a tin compound (Patent Document 35) and a compound of Sc, Mo, Mn, Bi, Te (Patent Document 36).
On the other hand, it is also attempted to improve the yield of aromatic carbonate by shifting the equilibrium to the product side as much as possible by designing a suitable reaction system. For example, a method in which by-product methanol is distilled off together with an azeotropic agent by azeotropic distillation in a reaction between dimethyl carbonate and phenol (Patent Document 37 and corresponding Patent Documents 38 and 39, Patent Document 40) and a method in which by-product methanol is removed by adsorption using molecular sieve (Patent Document 41 which is corresponding to Patent Document 42) are proposed.
Further, a method in which alcohol produced by the above-mentioned reaction is distilled off from a reaction mixture using an apparatus having a distillation column on the upper part of a reactor is also known (Examples of Patent Document 43 which is corresponding to Patent Document 44, Examples of Patent Document 45, Examples of Patent Document 46 and corresponding Patent Document 47, Examples of Patent Document 48 which is corresponding Patent Document 49, Examples of Patent Document 50 which is corresponding Patent Document 51), Examples of Patent Documents 52, 53 and 54).
More preferable methods include a method in which dialkyl carbonate and an aromatic hydroxy compound are continuously supplied to a multi-stage distillation column and continuously reacted in the column while continuously removing produced low boiling point components including alcohol by distillation and extracting generated products containing alkylaryl carbonate from the bottom of the column (Patent Document 55), and a method in which alkylaryl carbonate is continuously supplied to a multi-stage distillation column and continuously reacted in the column while removing produced low boiling point components including dialkyl carbonate by distillation and extracting generated products containing diaryl carbonate from the bottom of the column (Patent Document 56). These methods are the first disclosures of efficient and continuous production of aromatic carbonate. Similar continuous production methods have been thereafter filed, such as a method in which materials are brought into contact in a column type reactor to perform transesterification (Patent Documents 57, 58 and 59), a method in which a plurality of reaction vessels are connected in series (Patent Documents 60 and 61), a method in which a bubble column reactor is used (Patent Document 62) and a method in which a vertical reaction vessel is used (Patent Document 63).
Regarding industrial production of aromatic carbonate by these methods, methods for stable and long-term operation have also been proposed. Patent Document 64 discloses a method in which aliphatic alcohol is removed from a distillation column connected to a reactor so that the concentration of the aliphatic alcohol in the reactor is 2% by weight or less upon production of aromatic carbonate from dialkyl carbonate and an aromatic hydroxy compound, and it is described that stable continuous operation was achieved. The method of the publication is directed to avoid problems of precipitation of catalyst in the distillation column. Further, Patent Document 65 discloses a method which enables long-term stable operation by suppressing precipitation of catalyst by controlling the weight ratio of an aromatic polyhydroxy compound and/or a residue thereof to 2 or less relative to the metal component of the catalyst in a liquid matter in the system containing the catalyst.
On the other hand, the catalyst used in this reaction system is usually dissolved in a reaction mixture under reaction conditions, and has a higher boiling point than aromatic carbonate. Therefore, to obtain high purity aromatic carbonate from the reaction mixture, low boiling point components are first removed from the mixture and then diaryl carbonate of the high boiling point components is separated from the catalyst so as to purify the diaryl carbonate. It is known that the catalyst may be recovered and reused as a high boiling point component in that case, and deactivated components may be removed. Examples of such method of separating catalyst are described in Patent Document 66.
Titanium compounds are known to be an excellent catalyst (e.g., Patent Documents 67 and 68). However, when titanium based catalysts, for example, titanium phenoxide: Ti(OPh)4 or titanium alkoxide: Ti(OR)4, is used, there is a problem that diaryl carbonate which is the final product is colored with dark red as described in Patent Document 69. This is because, since titanium butoxide: Ti(OBu)4 typically used has a boiling point of 206° C. at 1.3 KPa and titanium phenoxide typically used has a boiling point of about 250° C. at about 27 Pa (No patent Document 1), they are also distilled off in a proportion corresponding to their vapor pressure upon separation of diaryl carbonate by distillation and therefore not sufficiently separated from the product. Further, as described in Patent Document 70, degradation of catalyst and deterioration of diaryl carbonate due to high temperature in separation of titanium phenoxide have been reported. Also, Patent Document 71 describes a problem that when Ti(OPh)4 is used and a mixture of starting materials and a catalyst is supplied to a continuous multi-stage distillation column to continue the reaction, the distillation column may be clogged. Thus, the well-known titanium based catalysts have problems that due to their high vapor pressure under reaction conditions, separation of produced diaryl carbonate is difficult, and the fraction at high temperature for a long period causes degradation of catalyst, clogging of the distillation column and deterioration of diaryl carbonate.
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